Retrieve value of QTableWidget cells which are QLineEdit widgets

Question

I create a QLineEdit,set a validator and put it on the table with this code:

ui->moneyTableWidget->setCellWidget(rowsNum, 1, newQLineEdit);

Then I've got another class for manipulating the table's data doing the sum of every value of a column. Here's the code:

int Calculator::calculatePricesSum(QTableWidget &moneyTableWidget){
    double total = 0;
    QWidget *tmpLineEdit;
    QString *tmpString;
    for(int row=0; row<moneyTableWidget.rowCount(); row++){
        tmpLineEdit = (QLineEdit*)moneyTableWidget.cellWidget(row,1);       
        tmpString = tmpLineEdit.text();
        total += tmpString->toDouble();
    }
    return total;
}

But the building fails with this error:

/home/testpec/src/nokia QT/MoneyTracker-build-simulator/../MoneyTracker/calculator.cpp:11: error: cannot convert ‘QLineEdit*’ to ‘QWidget*’ in assignment

Why this convertion error?

Another subquestion: passing the table as reference saves memory right? Could this be the problem? Im developing for a Nokia smartphone and I think passing the object by value is a waste of memory...(sorry if is a dumb question but I'm a little rusty with C++ and all the pointers stuff...)